1. Field of the Invention
The present invention relates to an improved processing circuit in which DC-voltage-output type sensors, such as thermocouples and thermopiles, are connected.
2. Description of the Prior Art
Generally, in a radiation thermometer using a thermopile, a processing circuit, as shown in FIG. 4, is adapted to cancel an input offset voltage at a preamplifier A by connecting a switch sw in parallel with a thermopile S (DC-voltage-output type sensor) which is connected to the preamplifier A. The switch is automatically ON/OFF controlled by a controlling means measuring C.
However, there are two kinds of offset voltage at the preamplifier A, that is, an input offset voltage and an offset voltage generated by an input bias current I.sub.B and a resistance r.sub.s of the thermopile S. In a processing circuit having the above described conventional construction, the former input offset voltage can be cancelled, but the latter offset voltage generated by the input bias current I.sub.B and resistance r.sub.s of the thermopile S cannot be cancelled. As a result, a problem exists in that a highly accurate measurement cannot be attained.
In order to reduce the influences caused by the offset voltage generated by the input bias current I.sub.B and the resistance r.sub.s of the thermopile S, the use of high-grade amplifiers, such as MOS-TOP type amplifiers, for preamplifier A has been considered. However, the amplifiers of this type generally produce a large amount of noise, creating a disadvantage in that it becomes very difficult to measure a minute voltage such as the output of the thermopile S. Additionally, productions cost are increased.